THICK SEDIMENT FILL IN UNAWEEP CANYON: IMPLICATIONS FOR THE HISTORY OF THE UNCOMPAHGRE UPLIFT, WESTERN COLORADO

Unaweep Canyon is a deep wind gap transecting the Uncompahgre Plateau. At its highest point (Unaweep Divide) the modern canyon floor lies 750 m above nearby rivers. Previous workers proposed various origins but without adequate knowledge of the valley-fill thickness or buried bedrock profile. Some assumed the canyon floor to approximate an ancient river channel and invoked combinations of differential erosion and over 400 m of late Cenozoic post-abandonment uplift of the Uncompahgre Plateau to account for the arched canyon floor. Other workers have suggested a glacial origin for the canyon.

Fill thickness was determined from 11 seismic refraction lines and 32 vertical electrical soundings spaced throughout the basement-walled part of the canyon (Oesleby, 1978). At Unaweep Divide, where the valley-floor width is approximately one km, seismic data indicate 330-395 m of unconsolidated valley fill with a velocity of 1.66-1.69 km/sec. Combined seismic and electrical data indicate valley fill thins northeastwards to 10-20 m at the canyon mouth 2 km SW of Cactus Park. Electrical data southwest of the divide to above the Narrows indicate the fill exceeds 130-240 m through the broad central part of the canyon. Data here are minima, limited by equipment (600-m-long Wenner array). Electrical data indicate the fill thins through the Narrows to 20-55m near the southwest canyon mouth.

The geophysical data are most consistent with 1) a V-shaped bedrock canyon produced by fluvial, not glacial, erosion; 2) subsequent partial fill with talus and alluvium (from canyon rim and walls) giving a U-shaped surface profile; 3) a longitudinal channel profile not modified by post-abandonment differential uplift or deformation; and 4) abandonment by piracy, not tectonic damming. A mean paleo-channel gradient of 7.6 m/km compares with gradients at nearby canyons: Colorado River in Glenwood Canyon (12.5 m/km); combined Colorado/Gunnison/Uncompahgre Rivers in Westwater Canyon (2.5 m/km); and the Gunnison River in Black Canyon (10m/km). The gradient is most consistent with a Gunnison/Uncompahgre River origin with abandonment driving upstream incision at Black Canyon.